Electrostatic printing is a printing technology in which electrostatic forces are used to form the image in powder or ink directly. Usually, ink is metered into an anilox, or gravure, roller such that the cells, or grooves, are partially filled. Ink refers to any material which is to be placed on a final substrate, and may include liquids, powders, and solid. To form an image, the ink is electrostatically pulled out of the cells in an image-wise fashion. Typically, metering rollers are used to meter the amount of ink applied to an anilox roller. An anilox roller includes a cylindrical surface with millions of very fine hollows, shaped as cells or grooves. Anilox and gravure are terms both referring to cylinders with small cells/grooves on the surface and may be used interchangeably. Technically, the term anilox is used more in flexographic printing and gravure is used in gravure printing. The gravure cells may usually be patterned in an image while the anilox cells may not be. Ink to be metered is filled in the cells. Doctor blades or wiping blades are usually used to clean the lands of the anilox roller. In doctor blade mode, doctor blades may be placed in an angle more than 90 degrees with respect to the blade moving direction. In wiping blade mode, wiping blades may be placed in angles less than 90 degrees with respect to the blade moving direction.
Existing technologies for electrostatic printing using anilox rollers have a number of drawbacks. Traditional cleaning using doctor blades may leave the cells full which leads to the problem of high background printing. The blades may be adjusted, but blades have inherent problems, including particle trapping, non-uniformity, speed limitations and cell pattern restrictions. For example, in a single blade system, there is an inherent conflict between the metering and cleaning requirements of the blade, as it needs to be soft enough to go into the cells or grooves, but hard or stiff enough to effectively wipe off residue ink from the lands. Another technique used a wiping blade mode, but this mode works only at slow speeds, as higher speeds increase the hydrodynamic pressure significantly.